The present invention pertains specifically to piezoelectric acoustic transducers, of either the single or multiple element type, wherein a ceramic piezoelectric disc is affixed in parallel overlying relationship to a piezoelectrically inactive acoustic radiation member so that electrical energization of the piezoelectric disc, with the consequent expansion and contraction thereof, will cause the acoustic radiation member to bend and produce sound pressure waves. It also may be utilized in piezoelectric transducers operating in a compressional mode.
In the bender type of piezoelectric transducer it is desirable to couple all, or as much as possible, of the stress created by energization of the piezoelectric disc to the inactive disc. To accomplish this the piezoelectrically active and inactive discs are generally rigidly attached together over the entire adjacent surfaces. Rigidly attaching the piezoelectric disc to the piezoelectrically inactive disc, which may be metal or the like, presents severe problems because of differences in thermal coefficients of expansion between ceramic and metal. The different thermal coefficients of expansion between the ceramic disc and the metal disc produce stresses in the ceramic disc which prevent efficient operation of the transducer and can, in severe cases, cause the ceramic disc to fracture. Further, static or slowly varying stresses in piezoelectric crystals, and especially soft ceramic compositions, cause a gradual permanent depoling of the material.